One of the world's most venerable technical problems is that of measuring displacement. Depending on the scale of the problem, any number of solutions have been proposed. With respect to large-scale measurements, rulers, tape measures and, more accurately, laser or other optical means have been used. For measurements of displacement that occur on a much smaller scale, optical interferometry or some other highly sensitive measurement method is often used.
Although there are many uses to which a measure of displacement might be applied, one that is of interest for purposes of the instant application is that of measuring the displacement that results from the application of force to a solid. That is, as is well known to those of ordinary skill in the art, strain results from the application of force to the external surface of a solid body. It is fundamental that given the force applied to the body and a measure of the amount of deformation, it is possible to calculate various physical parameters of the body including its shear modulus, Young's modulus, Poisson's ratio, etc.
However, many methods of calculating displacement under stress require expensive equipment and are not suitable for use in the field. Further, many of these measurement techniques are only applicable to measurement of a single axis of movement (e.g., longitudinally) and, thus, require multiple sensors to measure anything other than displacement along a single axis.
Optical methods are often preferred when making displacement measurements. Traditionally, methods of optical measurement of displacement fall into one of three categories: sensors that utilize interferometer techniques, sensors the employ optical gratings, and sensors that are based on the use of optical resonant cavities. As compared with electrical methods, optical methods do not require electrical wiring in order to transmit signals and, thus, they are not affected by electromagnetic interference which can cause measurement errors in electronic measurement methods. Further, electrical methods typically utilize a two element conductor to transmit signals and this conductor tends to be heavier than a corresponding capacity fiber optic line. This, of course, can pose a problem when the measuring device needs to move freely. As compared with mechanical measurement techniques, optical methods are preferred because of the increased accuracy that is possible. Further, optical methods are imminently suited for use in conjunction with a computer, whereas mechanical methods require some sort of translation/reformatting to make them into computer readable form. Finally, generally speaking optical methods do not traditionally require that contact be made between the sensor elements on the moving and stationary surfaces which can be an advantage in some settings.
However, optical measurement methods are not without their faults. This technology can require the use of large optical components such as beam-splitters, mirrors, lenses, etc., which can make application of this technology difficult outside of a laboratory.
Thus, what is needed an improved optical method and apparatus of measuring displacement which is not unduly complex and which does not require an inordinate amount of support equipment. Preferably, this apparatus will be suited for use in measuring strain/shear in solids.
Heretofore, as is well known in the measurement arts, there has been a need for an invention to address and solve the above-described problems. Accordingly, it should now be recognized, as was recognized by the present inventors, that there exists, and has existed for some time, a very real need for such a system.
Before proceeding to a description of the present invention, however, it should be noted and remembered that the description of the invention which follows, together with the accompanying drawings, should not be construed as limiting the invention to the examples (or preferred embodiments) shown and described. This is so because those skilled in the art to which the invention pertains will be able to devise other forms of this invention within the ambit of the appended claims.